Agile control of beams of electromagnetic radiation is important for many purposes. Directed energy weapons must direct their energy toward targets which may be moving. Also, systems for target detection by ranging and locating, such as radar or laser, directed communications systems involving moving vehicles, and controlled laser welding systems may all require that beams of electromagnetic radiation be directed in an agile and controlled manner. Instead of changing the direction, agile control may require that a beam be spoiled, or broadened, so as to be more likely to intercept a target.
Those skilled in the art of antennas and radiation know that such systems are reciprocal, and operate in the same manner in both a beam transmission and beam reception mode. Thus, descriptions of the operation of such systems may be couched in terms of either transmission or reception, with the other mode of operation being understood therefrom.
A scanning arrangement for control of an optical beam is described in U.S. Pat. No. 4,937,539, issued June 26, 1990 in the name of Grinberg et al. The Grinberg et al. arrangement includes at least one array of controlled liquid crystal phase shifters to provide beam deflection in a first direction. Control in a second direction is provided by cascading a second, mutually orthogonal phase shifter with the first phase shifter. The liquid crystal phase shifters include active portions through which light may pass and be controlled, and also includes regions with electrical conductors which are necessary to convey electrical control signals to various portions of the liquid crystal. These electrical conductors reflect the light which impinges thereon, with the result that portions of the liquid crystal phase shifter lying under the electrical conductor do not receive light, and are in effect inactive for the purpose of phase control of light passing through the phase shifting element. When a pair of cascaded arrays are used for the purpose of achieving scanning in two planes, the loss attributable to the inactive portions is also cascaded. In addition, the light reflected by the electrical conductors may create radiation patterns established by the array defined by the electrical conductors. This reflected energy may, in a reflective system, result in fixed grating lobes or sidelobes. For purposes of explanation, no distinction is made herein between the terms "energy" and "power", which later is formally the time rate of energy.
Mechanical beam scanning systems such as rotating mirrors or reflectors are effective for some purposes, as for example for surveillance such as radar volume surveillance. When targets must be tracked, however, mechanical systems may suffer from speed problems due to inertia, and reliability may be less than that desired.
U.S. Pat. No. 5,015,080, filed Mar. 26, 1990 in the name of Cassarly et al. describes a beam steerer in which an array of liquid crystal phase shifters is associated with at least one mechanically translated microlens array. Translation of a microlens array relative to an incident beam of energy as described therein may cause an unwanted second beam of energy due to overfilling of the aperture of a microlens of the array. The microlens array may be costly to fabricate, since it requires manufacture of lens elements in which the individual features may be smaller than a wavelength. The focusing effect of the lens may concentrate the beam of energy in a small portion of the liquid crystal. If substantial energy is involved, the concentration may result in control of the liquid crystal molecules by the electromagnetic field as well as by the control voltage, thereby creating a nonlinearity. In addition, thermal effects at the focus may cause deterioration of the liquid crystal. When a pair of liquid crystal phase shifters are to be cascaded, only one can be at the focus, with the result that the phase front in one liquid crystal phase shifter will be flat, while the phase front in the other will be curved. This in turn may complicate the control, introduce pointing inaccuracy, or cause beam spreading.
For small steer angle, high accuracy applications, an improved beam scanning and control arrangement may be desired.